This invention relates to the heteroepitaxial deposition of high quality, epitaxial, crack-free, low defect density films onto a thermally and/or lattice mismatched substrate. More specifically, it relates to the heteroepitaxial deposition of thick layers of compound semiconductor materials for subsequent use as substrates for further deposition. More specifically, it relates to the deposition of thick films of Gallium Nitride (GaN) and other Group III Nitrides (AlN, InN) and their alloys, for use as substrate material for further growth of device structures.
Gallium nitride (GaN) has been recognized as having great potential as a technological material. For example, GaN is used in the manufacture of blue light emitting diodes, semiconductor lasers, and other opto-electronic devices, as well as in the fabrication of high-temperature electronics devices.
One of the greatest challenges for the large-scale production of GaN-based devices is the lack of a suitable native GaN substrate or even thick layers of GaN. GaN is not found in nature; it cannot be melted and pulled from a boule like silicon, gallium arsenide, sapphire, etc., because at reasonable pressures its theoretical melting temperature exceeds its dissociation temperature. However, the fabrication of very high crystal quality, thin layers of GaN, and its related alloys, for use in electronic devices, requires that they he deposited homoepitaxially onto an existing GaN surface. Such high quality device layers cannot be directly grown heteroepitaxially, for reasons that are outside the scope of this invention.
The techniques currently in use for the fabrication of high quality GaN and related layers involve the heteroepitaxial deposition of a GaN device layer onto a suitable but non-ideal substrate. Currently such substrates include (but are not limited to) materials such as sapphire, silicon, silicon carbide, and gallium arsenide. All heteroepitaxial substrates present challenges to the high-quality deposition of GaN because of lattice and thermal mismatch. Lattice mismatch is caused by the difference in interatomic spacing of atoms in dissimilar crystals. Thermal mismatch is caused by differences in the coefficient of thermal expansion (CTE) between joined dissimilar materials, as the temperature is raised or lowered.
The most commonly used heteroepitaxial substrate for GaN deposition is sapphire (Al2O3), which has both a large thermal mismatch and a large lattice mismatch with GaN. For reasons unrelated to the scope of this invention, it otherwise possesses superior properties as a hetero-substrate. However, the large lattice mismatch results in films that have very high defect densities, specifically in the form of dislocations, which are especially undesirable from a device fabrication point of view. As with other epitaxial crystal growth processes, it is necessary to grow a buffer layer of GaN on the sapphire surface prior to the formation of device-quality layers. The buffer layer will vary, depending on device tolerance to dislocations, whether or not special growth techniques (such as growth through a mask pattern, use of low temperature buffer layers, etc.) are employed, as well as other factors. Typically, this GaN buffer thickness is several microns. Defect densities, however, predominantly in the form of dislocations, remain high (xcx9c1010cmxe2x88x922) resulting in diminished device quality. In addition, the sapphire substrate is not electrically conductive, and has poor thermal conductivity, limiting its heat sinking capabilities, further reducing device performance and complicating device processing.
A simple solution to this problem is to increase the GaN buffer layer thickness in the hopes that the dislocation density will decrease with increasing distance from the substrate interface. Furthermore, a thick GaN buffer layer offers improved electrical and thermal properties, which aids in the design and processing of devices. These very thick GaN buffer layers have been called xe2x80x98pseudo-substratesxe2x80x99. The difficulty in growing a sufficiently thick GaN buffer layer on sapphire, to act as a pseudo-substrate for subsequent device-quality layer growth, arises from the effects of thermal mismatch. Typically the GaN is deposited onto the sapphire at a temperature of between 1000-1100xc2x0 C.; as the sample cools to room temperature, the difference in thermal expansion (also contraction) rates gives rise to high levels of stress at the interface between the two materials. Sapphire has a higher coefficient of thermal expansion (CTE) than does GaN; as it cools, the mismatch at the interface puts the GaN under compression and the sapphire under tension. Up to a point, the amount of stress is directly related to the thickness of the deposited GaN, such that the thicker the film, the greater the stress. Above a film thickness of approximately 10 microns, the stress levels exceed the fracture limits of the GaN, and cracking and peeling of the film results. Cracks in this layer are much less desirable than high dislocation densities, and should be avoided because of the risk of their catastrophic propagation into the device layer during subsequent processing steps.
Referring to the drawings, FIGS. 1(a)-(b) schematically illustrate the prior art when deposition of a thick layer of GaN onto sapphire is desired. In FIG. 1(a), sapphire substrate 101 has a thick (greater than 10 microns) film of GaN 102 deposited onto it, at the growth temperature, which may be in the range of 1000-1100xc2x0 C. The actual method of deposition is not relevant to this invention. Because the film of GaN nucleates onto the substrate at this temperature, there is no thermal stress present. FIG. 1(b) shows the effects of the large temperature change as the sample cools to room temperature. In this FIG., sapphire substrate 101 is now under tensile stress and is bent concave with respect to the deposited film. If the stresses are great enough, cracks 103 may be present in the substrate. The epitaxial GaN 102 is under compressive stress, and is cracked, and may also peel away from or otherwise degrade the interface with substrate 101.
FIG. 2 schematically represents a series of steps involved in the conventional method for making a thick layer on a thermally and/or lattice mismatched substrate. Step 201 calls for the provision of a prepared substrate. This prepared substrate may be, for example, plain sapphire, chemically cleaned prior to use. Step 202 is the setting of process parameters and growth conditions for the growth of the thick, flat, high quality layer. Step 203 calls for the deposition of the thick layer onto the prepared substrate. The thickness of this layer is preferentially in the range of 10-400 microns. In step 204, the sample is cooled down to room temperature where it is removed, intact, from the reactor. The wafer is bowed due to the residual stress caused by the thermal mismatch between the epitaxial layer and the substrate. This stress also leads to the formation of many cracks in the thick layer and the substrate.
Therefore, a need exists in the art for arbitrarily thick, high quality, epitaxial, crack-free, low defect density films deposited onto thermally and/or lattice mismatched substrates and a method for depositing them.
The present invention provides a method to allow the growth of an arbitrarily thick, crack-free layer of GaN or related III-V compound or alloy onto a heteroepitaxial substrate. Additionally, it is another object of this invention to provide a method for the growth of such a film with a top surface having significantly fewer defects than are present at the interface of the GaN film and the heteroepitaxial substrate. Additionally, it is another object of this invention to provide a method for the production of a freestanding substrate of GaN, AlN, InN or their alloys.
It is another object of the present invention to provide a method for the production of a defect-rich first heteroepitaxial layer, which is essentially stress-free, and a subsequent second homoepitaxial layer, atop the first layer, that is of a very high crystal quality.
It is another object of the present invention to provide a method for the production of a defect-rich first heteroepitaxial layer, which is essentially stress-free, and the formation, in-situ, of a subsequent second homoepitaxial layer, atop the first layer, that is of a very high crystal quality.
It is another object of the present invention to describe a method for the production of 1) a highly defect-rich, stress-relieved first layer, followed by 2) the deposition of a second layer atop the first layer; where 2) acts to cover, isolate, and eliminate the effects of the defects in layer 1).
These and other objects, advantages, and features of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following, or may be realized and attained as particularly pointed out in the appended claims.